The presence of echo in telephone networks is known and understood.
Echos typically exist as a result of transmitted signals in a telephone network being coupled into a return path and fed back to the sender of the original transmitted signal. The most common cause of such a coupling is an impedance mismatch in a four wire to two wire hybrid. The impedance mismatch causes a reflection of some of the power transferred across the hybrid. A sender will receive the portion of a transmitted signal reflected by the impedance mismatch a short time after sending the original signal. Depending on the delay between the original and reflected signal, the reflected signal will create an echo perceived by the sender.
Echos can be avoided if the impedance of the hybrid exactly matches the impedance of the two wire circuit so that near perfect isolation of the two, two wire branches (or legs) of the four wire circuit can be realized. Such impedance matching is a time consuming manual process and is therefore not commonly used. Further, two wire circuits emanating from hybrids are typically switched connections such as customer sets, so that the impedance of the two wire line connected to the hybrid is rarely matched.
Numerous methods of reducing or eliminating echo are known. For example, telephone network design introduces specific amounts of net loss called via net loss ("VNL") that depends on the length of the link and typically creates end to end attenuation in proportion to the length of the circuit. An echo experiences twice as much attenuation as does a transmitted signal since it traverses twice the distance. Thus, the VNL may be relied upon to sufficiently suppress echos having thirty to forty milliseconds of delay. However, reliance on VNL is not appropriate for echos having longer delays.
Instead, echo suppressors and echo cancellers are typically deployed.
Echo suppressors operate on four wire circuits by measuring the speech power in each two wire portion (leg) of the circuit and introducing a large amount of loss in one leg when the power level in the other leg exceeds a threshold. Thus, a returning echo is essentially blocked by a high level of attenuation. However, an echo suppressor converts a full duplex circuit into a half duplex circuit with the energy sensed being the means of turning the circuit around. As a consequence, echo suppressors tend to clip beginning portions of speech segments. For example when a party at one end of the connection begins talking at the tail end of the other parties speech the echo suppressor does not have time to reverse directions.
A further and much preferred method of reducing or eliminating echo is echo cancellation. An echo canceller operates by simulating the echo path to subtract a properly delayed copy of a transmitted signal from the received signal to remove or cancel the echo component.
In a digital telephone network, echo cancellation is usually accomplished by applying digital signal processing ("DSP") techniques to pulse code modulated ("PCM") voice data. Echos are cancelled close to the point of reflection so that the delays the echo canceller must simulate are minimized.
Known DSP echo cancellation algorithms, as for example disclosed in U.S. Pat. No. 5,343,522, use call data from a far end caller (ie. call data without echo) and a signal traveling in the opposite direction, including a near end echo, to form a suitable adaptive filter. The filter is modeled by correlating the original signal with the return signal containing echo. The formed filter is suitable for removing the echo component of the return signal without noticeably modifying the remainder of the signal. The filter is updated periodically and is applied to all signals along the return path, destined to the speaker at the far end.
As will be appreciated, the initial formation of a suitable filter takes some time in order to effectively correlate the return signal portion including echo to the originating signal. An echo canceller that has been properly matched to a particular call is said to be "converged". The exact amount of time for convergence will vary from call to call depending on the nature of the echo and the nature of the call. For example, in the presence of an additional signal in the return path, signal correlation and hence echo canceller convergence is difficult, if not impossible. Therefore cancellers converge most effectively in the absence of signals originating at both ends of a call (often referred to as "double talk"). In the absence of significant "double talk", formation of a suitable filter may be much simpler and less time consuming. Present DSP echo cancellation algorithms typically require approximately two to three seconds to form a suitable filter for most calls.
Physically, echo cancellers are usually hardwired in series with trunks connecting telephone switching systems. As well, echo cancellers are typically installed on T1 lines, having twenty-four DS0 channels. As a result, echo cancellers are often bundled in units capable of providing echo cancelling service for a T1 trunk having twenty-four voice channels.
This hardwiring of echo cancellers leads to the problem that a failed canceller may be tantamount to a failed trunk. Some suggested solutions to this problem have involved the use of redundant hardware or hardware bypasses in the event of failure of the echo cancellers. Redundancy, however, is typically limited so that multiple hardware failures cannot be compensated. Moreover, hardware bypasses eliminate the provision of echo cancellation on at least one channel and typically on an entire trunk.
A much improved method of providing echo canceller redundancy is disclosed in copending U.S. patent application Ser. No. 08/851,026, the contents of which are hereby incorporated by reference. Basically, the disclosed method employs a communications switching system capable of switching individual calls to and through individual echo cancellers, in a pool of echo cancellers, as required. In the event a single echo canceller fails, this system could switch a call in need of echo cancellation to another available canceller. The echo cancellers are formed as part of resource modules ("RMs") forming part of a modular communications system. For design reasons, an RM dedicated to providing echo cancelling (known as an echo canceller resource module "ECRM") in the disclosed system typically has two hundred and eighty eight cancellers. Each canceller cancels echo in one direction in a single call. As a result a single ECRM may be associated with up to two hundred and eighty eight calls at one time.
While such a system may be capable of switching individual calls between available echo cancellers across ECRMs in the event of an echo canceller failure, replacement of an entire ECRMs is often desirable. Such replacement may be performed dynamically while the system is in operation. All calls processed by an active ECRM may be switched through a redundant ECRM. That is, an active ECRM is logically disconnected from the system and a redundant ECRM is connected. The dynamic replacement of an active (ie. in-use) ECRM with a redundant (ie. inactive) ECRM is referred to as "sparing" of that ECRM.
However, when calls are switched from echo cancellers on an active ECRM to echo cancellers on a redundant ECRM each newly used echo canceller will need to "converge". As a result, the switching from an active echo canceller to a redundant echo canceller effectively removes echo cancellation from the call for the amount of time required for convergence. In the event of a failed echo canceller, this delay is quite tolerable in view of the benefits provided by replacing the failed canceller.
However, when all calls on a single ECRM are switched to a redundant ECRM, all redundant echo cancellers on the redundant ECRM will need to converge, whether or not the corresponding active echo canceller on the active ECRM has failed. This, of course, will cause the presence of echos for a short period, on all calls associated with the active and redundant ECRMs.
The present invention attempts to overcome some of the disadvantages associated with present methods of replacing or "sparing" echo cancellers in a telephone communication switching system.